36 Congenital Night Blindness 



Although the explanation here offered to show how night-blindness 

 is probably inherited in this instance is on the whole rather satisfying, 

 it does not completely accord with the data shown in the pedigree. 

 Perhaps the most serious discrepancy is encountered in connection with 

 the apparent non-inheritance of the capacity to transmit night-blind- 

 ness on the part of the sisters of night-blind men. According to 

 expectation there should be in F^ or Fr„ assuming that there has been 

 no consanguineous marriage, equal numbers of night-blind carrying and 

 of normal females, but the pedigree seems to show that none of the 

 progeny of the five married sisters of C. D. Uzzell and E. B. Uzzell 

 (night-blind males of the third generation) show a trace of night- 

 blindness. Whether sisters of night-blind males can transmit the defect 

 will doubtless be determined in the present generation. If it should 

 prove that night-blindness cannot be inherited for two consecutive 

 generations in the female line, it would be a fact of unusual significance. 

 It would seem to be a case in which a prolonged association of the 

 defective X element with the normal X element in the germ plasm of 

 two generations of females might serve to correct the defect in the 

 former and thus eliminate night-blindness from these particular lines. 

 The value of this idea depends on whether or not the findings in the 

 pedigree are substantiated by future events. 



In common with other writers who have reported pedigrees of this 

 sort I find a serious discrepancy in the ratio of males and of females, 

 and in that of affecteds and unaffecteds. Of the 36 offspring of the 

 night-blind carrying daughters of night-blind men, 22 are males and 

 14 females instead of the expected 18 : 18. Perhaps this discrepancy 

 is within the range of probable error. But the fact that of these 

 22 males 17 are affected and only 5 unaffected can hardly be so 

 interpreted. An excess of affecteds, however, is found quite generally 

 in haemophilia and in certain less known inherited affections, and it 

 may well be, as Bateson suggests, that in these cases there is present 

 some unknown disturbing factor that distorts the normal Mendelian 

 ratios. 



The association of night-blindness with other optic defects. 



Nettleship and others have noted that the sex-limited type of 

 night-blindness is almost invariably associated with myopia. He even 

 finds it sometimes associated in inheritance with certain mental defects. 

 The present pedigree is in agreement with the conditions described by 



